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I'm hoping someone has some ideas for me to try. I have a 2011 Toyota Sienna LE. The interior lights haven't worked in quite some time. I checked and the fuse for the interior lights was blown. I finally got around to getting new fuses and trying to replace the blown fuse. New fuses blow immediately. They even blow when the overhead buttons are set so the interior lights wouldn't come on when opening the door. I popped the overhead console out to check for anything obviously wrong up there, but I did not see anything abnormal.

Any ideas for why this could be happening? I'm kind of assuming there's a short somewhere but I don't know where to start with pinning that down. Any help would be appreciated.
 

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Additional info: I may or may not have had mice (I did...) in my van. I have no idea how they got in but I did hear them scratching around in the ceiling at one point. I set traps and caught 5 mice.
 

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I would imagine that mice had chew the wire somewhere and that creates a short.

I would strongly recommend - DO NOT even try to turn the light on until you find and fixed the issue. Otherwise, you're exposing your van to catch the fire instantly.
 

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I would imagine that mice had chew the wire somewhere and that creates a short.
Agreed. Now the challenge is finding where. Might be multiple locations. Fun stuff! More on how, below.

I would strongly recommend - DO NOT even try to turn the light on until you find and fixed the issue. Otherwise, you're exposing your van to catch the fire instantly.

Nah. That's what fuses are for. That's what protects against fire. If it blows instantly, there is no more current flowing and no more danger. That's why you NEVER NEVER NEVER replace a blown fuse with a larger fuse. Not even to test, or to limp home.


Ryan, this is not conceptually difficult, but it's tedious work finding that short. It can take a while. If you take it to a shop, they will not give you an estimate, because they can't. It's just pay by the hour until they find the problem. Since few mechanics really understand electricity, they will most likely be shotgunning and it will be expensive. (If a shop does give you an estimate on this kind of problem, walk away! They have no clue what they are doing!)

If you want to do it yourself, here are a few tips:

First and foremost, get a notebook! You need to do a lot of testing, and you WILL NOT remember what you did and what the results were. Write it down! This notebook is a great place to record your plan. "Plan?" Yes, you are going to make a plan, and follow it. You are not going to just jump around, guessing here and guessing there. More on that later.

You'll need a meter, AND a test light. A cheap meter will do for this.

Get a bunch of fuses of the correct size, OR SMALLER. Smaller is actually better - you want to blow them quickly. You are going to go through a bunch, so get cheap ones. I would not use no-name Chinese knock-off fuses in my car, normally, but for this, they are perfect. You can find them in bulk online. Get at least a 10 pack, and more is better. OR you can use a buzzer. More on that below.

Now, you are going to be changing fuses a lot. I'm not sure where your blowing fuse is located. According to this, it's in the fuse block in the engine compartment. If so, that's easy. If it's up under the dash, that's another story. IF that's the case, use the buzzer trick.

Now, wherever the fuse is, you might want to cheat a bit. Get a buzzer and a couple of male .110 quick disconnects. Crimp the connectors to the ends of the buzzer's wires and insert them in place of the fuse. Now, instead of a fuse blowing, the buzzer will sound. Once you have it fixed, the buzzer will stop. (The wires are pretty fine, and wont' fill up the connector well. You'll need to strip back about a full inch of insulation, and fold the wire over two or three times before you put it into the connector to crimp. Twist it tightly, first.) These buzzers draw a very tiny amount of current (12 mA for the ones I linked above) so you are not going to burn anything up. You'll just be annoyed by the sound! :D

Okay, with everything ready to go, remove ALL of the interior light bulbs. It's entirely possible that one of them is shorted internally. It happens. If the buzzer shuts off when you take them out, there ya go. Just start putting them back one by one until you hear the buzzer again.

If not, leave them out, and now the fun begins. More later, as I have to run now.
 

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Okay, you have your buzzer in place (or your pack of fuses ready) and you have verified that the bulbs are good. But that buzzer won't shut up! (I'm just going to refer to the buzzer for simplicity. If you are using fuses, you'll need to check for a blown fuse at each step instead of listening for the buzzer.)

Now it's time to apply some thinking. If you can get a wiring diagram, your life will be easier. You want to do what is called, "circuit splitting". This is a technique we use in electronics to speed up finding a fault in a circuit. Basically, you divide a circuit in half logically. Not physically, logically. Or you might say, functionally. We don't care if you split it a certain distance from some point. We want to split it between functions.

Let's make up a simpler circuit to illustrate the concept. Let's say we have a battery, a fan motor, a switch, and some connectors.
  1. [BATTERY]
  2. <-> (connector)
  3. <-> (connector)
  4. [SWITCH]
  5. <-> (connector)
  6. <-> (connector)
  7. [MOTOR]

(Again, we don’t care at all about the physical distance between the components.)

So a rough halfway point is #4, the switch. If we disconnect the switch, we can then test the first half of the circuit. That’s all the wiring from the battery to the switch, plus connectors 2 and 3. If there is no problem there, we know the problem is downstream of there. Maybe the switch itself, maybe farther down. So let’s reconnect the switch, and find another halfway point in the second half. Maybe Connector 5. Disconnect it, and test. If the buzzer sounds, your short is between that connector and the last point we tested, the switch. If not, it’s after connector 5. [EDIT: I goofed and said 3 here. I fixed it.]

So you keep splitting the circuit to narrow down the area where the problem lives.

As you can see, a wiring diagram makes this a LOT easier! Your circuit probably has branches, which makes it more complicated, but the concept is the same.

This approach will work a lot faster and more certainly than the usual, “try this, try that, guess again, now what?” approach.

Holler if you have questions!

EDIT: Another useful tool is an endoscope. You can use it to peer around under the headliner, down inside pillars, and other nooks and crannies where wiring lives. You might just see some munched wires.
 

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Wow BlueRidgeMark - that's some pretty thorough stuff! Thank you!

Just FYI - I'm paralyzed (arms work, legs don't) and this is a 2011 Toyota Sienna LE with Braun conversion for wheelchair access. It has lowered floors, no middle row seats, and a ramp that folds out the passenger side sliding door. I'm not afraid to get into electrical things but I really haven't done any electrical troubleshooting like this before. I did install a touch-screen head unit (including steering wheel controls converter/adapter to retain factory controls), replaced all 6 speakers, and put a subwoofer in back (with a custom sub box I built). And before anyone goes jumping all over that info as being the cause of my problem - the fuse for the lights was blowing long before I did anything to the stereo system. My dome lights haven't worked in years... I'm just now getting around to doing something about it.

I had left a mouse trap in my van on the floor, next to a seat (just in case mice returned), and sure enough - I started catching them again a couple days ago... and have no idea how the mice are getting in and I'm good about not leaving food in the van...

The one bit of troubleshooting I did do was that I un-pressed the overhead button that makes lights go on when a door is opened. I also made sure none of the other interior light switches were set to the "on" position. I then tried replacing the fuse, and it instantly blew.

I hope to be able to try some of these suggestions when I get some time this weekend. I'll post updates as I can. Thanks!
 
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